The invention concerns improvements in wideband power amplifiers for communication of RF signals and in particular to power amplifiers employing envelope elimination and restoration (EER) of the type disclosed in U.S. Pat. No. 6,300,826 to Mathe et al.
An EER amplifier separates an incoming complex signal, A(t)ej(ωt+φ(t)), into two channels, the amplitude channel carrying A(t) and phase channel carrying ej(ωt+φ(t)); where φ(t) is the time-dependent phase of the incoming signal, ω is the carrier frequency and A(t) is the amplitude or envelope. The envelope A(t) has frequency bandwidth from DC to the maximum base band frequency, while the phase channel retains the original carrier frequency and contains only the phase information after the amplitude or envelope A(t) is eliminated. A highly linear power amplifier, such as a class AB amplifier, amplifies the signal in the phase channel. The envelope is restored in the amplified signal by an EER modulator that modulates the bias power of the linear power amplifier in accordance with the envelope A(t). The envelope A(t) is a very wideband signal, containing frequency components ranging from the maximum base band frequency down to D.C. The EER modulator must have the capability of faithfully reproducing and amplifying all components in this wideband signal.
This extremely wideband modulator, working as a power supply for the power amplifier, must have high power gain, high efficiency and broad frequency response, because it directly affects the overall system power efficiency. To achieve these goals, two amplifiers are employed: one amplifier has a low to high frequency response, but low efficiency and low power; the other covers from D.C. to about 50% bandwidth, and delivers high current with high efficiency. The high frequency amplifier amplifies the highest frequency components of the envelope signal while the power amplifier amplifies the remaining (medium frequency, low frequency, and D.C.) components. The power amplifier must be capable of generating high current at low frequencies, while the high frequency amplifier must be capable of replicating high frequency components in the incoming signal (leading and trailing edges, spikes, and the like). The two amplifiers are therefore very different in their output characteristics. The difficulty arises in combining the outputs of the two amplifiers so as to obtain a faithful amplified reproduction of the envelope signal A(t). The above-referenced patent to Mathe et al. discloses one technique in which a feedback loop governs the output of the power amplifier based upon the output of the high frequency amplifier.
The technique for combining the outputs of the two different amplifiers disclosed in the above-referenced patent to Mathe et al. has been found to be inadequate. Therefore, there is still a need for a way of combining the outputs of the high frequency amplifier and the power amplifier to attain a faithful amplified reproduction of the input signal.
In considering this problem, I recognized that the disparity between the output impedances of the high frequency and power amplifiers was a source of difficulty. Specifically, the high frequency amplifier is typically a low output impedance voltage source, while the power amplifier is typically a PWM (pulse-width modulated) switching mode, high efficiency with a relatively high output impedance source. I also recognized another difficulty with the Mathe et al. technique is the use of the high frequency amplifier output current to govern the output of the power amplifier. The real need was to govern the output of the power amplifier in such a way as to minimize differences between the power amplifier's output and the actual envelope signal A(t), while at the same time solving the problem of the disparity between the output impedances of the two amplifiers.